1. Field of the Invention
The present invention relates to a brake disc used for a general vehicle, particularly to a brake disc for a two-wheeled vehicle. The present invention relates to a brake disc stably having a necessary hardness as a brake in an as quenched state after working, excellent in rusting resistance and softening resistance during use, and produced from a martensitic stainless steel.
2. Description of the Prior Art
A brake disc of a two-wheeled vehicle is required to have properties such as abrasion resistance, rusting resistance and toughness. Although the abrasion resistance of the brake disc is generally improved as the hardness is increased, a so-called brake noise is produced between the brake and the pad when the hardness is too high. The hardness of the brake is, therefore, required to be 35.+-.3 HRC (Rockwell hardness C scale). To obtain the desired hardness and rusting resistance, a martensitic stainless steel is used as a brake disc material.
In prior art for the brake disc material, SUS 420 J1 (13Cr-0.2C) type martensitic stainless steel has been quenched and tempered to have the desired hardness, and used as a brake disc. However, the prior art requires two heat treatment steps of quenching and tempering, and, therefore, has problems of productivity and price. In addition, the prior art has the limited heat treatment condition because it is difficult to obtain a necessary hardness after tempering in the quenching temperature ranges. Accordingly, demand for a martensitic stainless steel which can be used for a brake in an as quenched state has been growing to save production steps and energy.
In order to meet the requirements as mentioned above, Kokai (Japanese Unexamined Patent Publication) Nos. 55-21566 and 57-198249 disclose steel compositions in which the amounts of C and N are reduced and Mn, an austenite-forming element, for compensating a narrowed hardenable temperature region due to the shrinkage of an austenite temperature region caused by reducing the amounts of C and N is added, and Which consequently stably shows the desired hardness in an as quenched state brought about by hardening from a wide temperature range compared with conventional steels. However, since the steels contain Mn in respective amounts of 1.0 to 3.5% by weight and 1.0 to 2.5% by weight, they each contain a large amount of MnS which impairs the rusting resistance. Accordingly, it must be concluded that the quenching stability is given to the steels in preference to and at the cost of the rusting resistance. To improve the drawback as mentioned above, Kokai (Japanese Unexamined Patent Publication) No. 61-174361 discloses a steel composition which is prepared by adding Mn in a decreased amount while Cu and N as elements not impairing the rusting resistance are added instead in respective amounts of 0.5 to 1.2% by weight and 0.03 to 0.07% by weight to ensure the quenching stability. Although the steel keeps the quenching stability and rusting resistance as desired, it has a N content as high as at least 0.03% by weight. Consequently, there is a fear that when the steel is tempered by heat generation caused by braking during use of the brake disc, fine nitrides may precipitate to lower the toughness and that the brake disc may not perform its function. Moreover, it is said that the brake disc is sometimes heated to 500 to 600.degree. C. as a result of heat generation caused by braking. The steels which have heretofore been proposed as mentioned above have, therefore, a problem that softening by tempering is unavoidable when the steels reach temperatures in the range mentioned above.